Power converters are broadly used in the computer and telecommunications industries to provide local power in a large number of applications. Synchronous rectification, using primary and secondary controllable switches, is widely used to improve the efficiency of a converter over one employing diode rectification. Diode rectifiers provide a conducting or forward voltage drop of about 700 millivolts as may be compared to synchronous rectifiers that provide a conducting voltage drop of only about 50 millivolts. Therefore, the rectifier power loss employing synchronous rectification is about an order of magnitude less than diode rectification. However, diode rectification may be used during a start-up mode of the synchronous converter.
One example of using diode rectification in the start-up mode occurs when the synchronous converter is used in parallel with another operating voltage source that provides a preexisting or pre-bias voltage on the output of the synchronous converter. This pre-bias voltage condition would cause the synchronous converter, employing synchronous rectification, to both source and sink load current during start-up until its output voltage reached the pre-bias voltage level. This sinking of load current is both highly undesirable and unacceptable by end users of the power converter, since it causes unpredictable or unacceptable current loads on the operating parallel voltage source thereby subjecting it to overload or failure. To solve the pre-bias voltage issue, the secondary controllable switch has to be disabled at the first stage of start-up, and secondary diode rectification used instead.
Another problem is typically encountered when converter operation transfers to the synchronous rectification mode. If the secondary rectifier suddenly transfers from the diode mode to the synchronous rectifier mode, this hard transition results in an output voltage spike having a polarity that is dependent upon converter load current. An overshoot may occur with high loads and an undershoot may occur with light loads. Both of these effects prevent a desired monotonic rise in output voltage and may also create an undesirable reverse current issue. Each of these effects results in a deleterious effect on the end system.
Accordingly, what is needed in the art is a way to transition from a diode rectification mode to a synchronous rectification mode without substantially affecting the output voltage.